ASTROPARTICLE PHYSICS LECTURE 3
1
Susan Cartwright
University of Sheffield
HIGH ENERGY ASTROPARTICLE PHYSICS
2
Acceleration Mechanisms
Sources
Detection
ν
NEUTRINO DETECTION


Neutrino cross-section rises with energy
Only UHE neutrinos (>1015 eV or so)
interact with reasonably high probability
(such that Earth is opaque to them)
Connolly, Thorne & Waters, hep-ph/1102.0691v1
ν
3
NEUTRINO DETECTION (PENETRATING NEUTRINOS)

Mostly rely on detecting the charged lepton produced in
CC interactions
at lowest energies (solar neutrinos), also elastic scattering
(ν + e → ν + e) and NC reaction on deuterium (ν + d → ν + p + n)
 note that at solar neutrino energies μ and τ cannot be
produced by CC, so νμ, ντ only seen in NC (e.g. SNO)


Some early experiments using tracking calorimeters, but
water Cherenkovs now standard practice
can obtain large effective volumes by instrumenting natural
bodies of water/ice
 particle identification by ring morphology at low energies,
shower shape at high energies

4
NEUTRINO DETECTION BY WATER CHERENKOV
Transparent
medium
(water/ice)
PMTs
Cherenkov light
μ
CC interaction
νμ
Effective volume
increases with
energy
5
BACKGROUNDS

Cosmic ray muons
Go deep
 Look down



therefore, northern hemisphere
telescope sees southern sky,
and vice versa
Atmospheric neutrinos
one man’s signal is another’s
background!
 irreducible, but steeper spectrum
than high-energy astrophysical
neutrinos

6
PARTICLE ID: SUPER-KAMIOKANDE
muon:
sharp ring
electron:
fuzzy ring
7
PARTICLE ID: ICECUBE
νμ
νe
“double-bang” ντ event: initial signal from
CC interaction, later one from τ decay
8
Halzen & Klein, Rev. Sci. Inst. 81 (2010) 081101
ντ
HIGH-ENERGY NEUTRINO TELESCOPES
9
LAKE BAIKAL
1. Central core (NT200) with 96 pairs of
OMs on 8 strings
2. Outer ring with 3 additional strings each
equipped with 6 OM pairs
3. Lasers for calibration
Each OM
equipped with
37-cm PMT
10
ANTARES
2475 m deep, 42 km off Toulon
885 OMs arranged in triplets on 12 lines;
each OM equipped with 10” PMT
Acoustic transponders for position
monitoring
LED and laser optical beacons for
calibration
11
ICECUBE
Much the largest existing detector,
instrumenting 1 km3 of Antarctic ice.
Precursor, AMANDA II, very similar to
ANTARES in size and sensitivity.
12
MEDIUM PROPERTIES
Lake Baikal
Mediterranean
(ANTARES)
Antarctic ice
Absorption
length (m)
20−24
50−70 (blue)
~100
Scattering length
(m)
30−70
230−300 (blue)
~20
Depth
1370
2475
2450
Property
Noise
Retrieve/
redeploy
Quiet
Yes
40K,
bioluminescence
Yes
Quiet
No
Long scattering length for ANTARES implies better angular resolution;
long absorption length for IceCube implies sparser instrumentation.
Quiet environments imply potentially useful data from singles rates.
13
BACKGROUND IN ANTARES

Three components
steady background of
~60 kHz from 40K
 slowly varying contribution from bioluminescence, probably
bacterial
 short bursts of strong bioluminescence, probably from larger
organisms


Correlated within a single storey, but not over long distances
minimal influence on tracking efficiency
 does probably preclude use of singles rate, e.g. for detection of
low energy neutrinos from supernova

14
LIGHT TRANSMISSION IN ICECUBE
Scattering is a consequence of dust layers in the ice—function of global
climate, level of volcanic activity, etc. “Dust logger” measures reflected light
from artificial light source just after drilling: measure scattering with few
mm vertical resolution. Note additional contribution from bubbles at
shallow depths (<1400 m); IceCube deployed below this level.
15
ANGULAR RESOLUTION
Moon’s shadow in CR muons,
measured by IceCube
Expected IceCube angular
resolution ~0.5°
16
EXPECTED FLUXES

Expect high-energy
astrophysical neutrinos
to be produced in proton
interaction cascades
therefore, observed CR
flux implies upper bound
on neutrino flux (WaxmanBahcall bound: Phys. Rev. D59 (1998) 023002)
 argument goes as follows:





from observed CR rate, deduce that the amount of energy emitted by
astrophysical sources in the form of UHE CRs (1019 – 1021 eV) is of order
1037 J Mpc−3 yr−1.
assume that CRs lose some fraction ε of their energy through pion
photoproduction before escaping the source
fraction of proton energy carried by neutrino produced in this way is about
5% independent of proton energy, so neutrino energy spectrum follows
scaled-down version of proton spectrum
resulting bound is Eν2φν < 2×10−8 GeV cm−2 s−1 sr−1 for 1014−1016 eV ν
17
RESULTS
For 3 years of data-taking: 37 events above 100 TeV,
on background of about 15
Flux at 100 TeV:
(6.7±1.2)×10−18
GeV-1 s-1 sr-1 cm-2
Spectral index
2.50±0.09
No significant
point sources or
correlations with
other data
18
RESULTS
No neutrinos
from GRBs
No GZK
neutrinos
No point sources
(combined
IceCube/ANTARES
analysis: largest
significance 0.7σ)
19
Example IceCube events
20
TAU-NEUTRINO DETECTION BY AIR SHOWERS
Earth-skimming ντ interacts in Earth’s crust to produce τ
 τ decay in atmosphere initiates characteristic air shower

shower appears to be in early stage of development—typical
horizontal shower is “old”
 searched for by Auger—no signal (PRD 79 (2009) 102001)

22
HIGH ENERGY ASTROPARTICLE PHYSICS
23
New Detection Techniques
RADIO-FREQUENCY DETECTION OF AIR SHOWERS AND
NEUTRINOS

Geosynchrotron emission (10−100 MHz)
synchrotron radiation from air-shower particles gyrating in Earth’s
magnetic field
 advantages over fluorescence:




disadvantages:



very high duty cycle (only wiped out by thunderstorms)
low attenuation (so, large effective area)
interference (need radio-quiet sites)
high threshold (1017 eV)
Radio Cherenkov (Askaryan effect) (0.1−2 GHz)

Cherenkov emission from neutrino-induced showers because of net
negative charge


initially neutral shower develops ~20% negative bias because of
annihilation of e+ and additional e− from Compton scattering etc.
requires dense, radio-transparent medium
 not air, not water
24
GEOSYNCHROTRON EMISSION


Studies run in association with Auger and KASCADE CR ground arrays
A declared key science goal of LOFAR Collaboration
25
LOFAR
Low frequency radio array
based in the Netherlands
Mostly a radio astronomy
facility, but good prospects for
radio detection of UHECRs (see
LOPES/KASCADE).
Also good for gravitational wave
follow-up (excellent wide-field
coverage)
26
LOPES/KASCADE
KASCADE:
scintillator-based
ground array
 LOPES (LOFAR PrototypE Station)

initially 10, now 30, low-frequency
RF antennas triggered by
KASCADE “large event” trigger
 KASCADE reconstruction
provides input to LOPES recon:

core position of air shower
 its direction
 its size

27
LOPES/KASCADE

First detection: January 2004
strong coherent radio signal
coincident with KASCADE shower
 reconstruction location agreed with
KASCADE to 0.5°


Extensive data sample now accrued

technique works well and suggests full
LOFAR array
should be
excellent
CR detector
28
LOFAR AS A COSMIC RAY DETECTOR

Small scintillator-based airshower array (LORA) set up in
LOFAR core
plastic scintillator detectors from
KASCADE, set up in 5 sets of 4
 estimated energy resolution
~30%, angular resolution ~1%
 combined running with LOFAR

Thoudam et al., astro-ph/1102.0946v1
29
AUGER/AERA


Preliminary studies using a few
radio antennas at the Auger
site gave promising results
Plan to instrument 20 km2 near
Coihueco fluorescence
telescope with 160
autonomous self-triggering
radio antennas
5000 events/year expected,
1000 above 1018 eV
 124 stations deployed so far

30
AUGER/AERA/AMIGA EVENT DETECTION
Shower depth
determination by AERA
and fluorescence
Event detected by all four Auger subdetectors
31
ASKARYAN EFFECT

Effect demonstrated in sand(2000),
rock salt (2004) and ice (2006)


all done in laboratory at SLAC
Applications to neutrino detection

using the Moon as target
GLUE (detectors are Goldstone RTs)
 NuMoon (Westerbork array; LOFAR)
 RESUN (EVLA)


using ice as target
FORTE (satellite observing Greenland ice sheet)
 RICE (co-deployed on AMANDA strings, viewing
Antarctic ice)
 ANITA (balloon-borne over Antarctica, viewing
Antarctic ice)

32
ASKARYAN EFFECT: ANITA
33
ASKARYAN EFFECT


Jaeger et al.,
Astropart.
Phys. 34
(2010) 293
ANITA observed UHECRs
(geosynchrotron signal)
Nobody saw neutrinos (sadly)
34
ACOUSTIC DETECTION (SHOWERING NEUTRINOS)

UHE (>1 PeV) neutrinos interact fairly readily

on entering dense medium (water) they will initiate shower
this dumps energy in a thin cylinder (~20 m × 20 cm)
 resulting pressure pulse spreads out from this cylinder in thin
“pancake” perpendicular to incoming neutrino direction
 produces characteristic bipolar acoustic pulse which can be detected
by hydrophone array


advantages
extremely long attenuation length (several km)
 very large volume can in principle be instrumented with relatively
small number of hydrophones
 hydrophone technology well established in underwater applications
 can use off-the-shelf hardware


disadvantages

the sea is a very noisy place
 identifying signal very challenging
35
PRINCIPLES
36
EXPERIMENTS

ACORNE


AMADEUS


part of NEMO (NEutrino Mediterranean Observatory, not
Neutrino Ettore Majorana Observatory!)
SAUND-I and SAUND-II


codeployed with Baikal-200
ONDE


codeployed with ANTARES
Lake Baikal


UK feasibility study using military hydrophone array off Rona
in Bahamas, originally using military array, now extended
SPATS

at South Pole, associated with IceCube
37
ACORNE

MoD hydrophone array off NW
coast of Scotland
successful R&D project showing
feasibility of technique
 array geometry not optimal
(not designed for neutrinos!)

Example of
background
source—
dolphin clicks!
38
AMADEUS

Acoustic storeys
added to ANTARES
strings


R&D project
comparing different
hydrophones
feasibility study for
KM3NeT
39
SPATS

Acoustic sensors on strings
deployed in association
with IceCube

very good at detecting
IceCube drilling and water
storage activities!
40
ACOUSTIC DETECTION: SUMMARY

Experiments so far are R&D
projects/feasibility studies


limits not competitive with
radio at present
Future strategy mostly
co-deployment with
large optical Cherenkovs
improves high-energy
sensitivity
 likely future direction: super-hybrid experiments with optical
Cherenkov, acoustic and radio elements, plus air-shower array if
appropriate


most nearly realised at South Pole with IceCube/IceTop/RICE/SPATS
41
NEUTRINO DETECTION: SUMMARY

High-energy neutrinos could provide information on
acceleration processes in high-energy astrophysics
 GZK cut-off in cosmic rays
 dark matter (see next lecture)


Detection still in infancy


only IceCube probably large enough to collect statistics
Various promising techniques
water Cherenkov at lower energies
 radio and possibly acoustic at high end


Hybrid experiments feasible at many sites
42